Thanks! My textbook has hints at the back of the book, which says that I should establish that 3^20=1(mod 25), and that 3^2=1(mod4), and that finally, 3^20=1mod(100). Your solution makes much more sense . By the way, how do you calculate phi(m) in general?

Thanks! My textbook has hints at the back of the book, which says that I should establish that 3^20=1(mod 25), and that 3^2=1(mod4), and that finally, 3^20=1mod(100). Your solution makes much more sense . By the way, how do you calculate phi(m) in general?